The sun's out and summer is well and truly upon us. And so, as we look forward to that summer holiday, we wake up to discussion around one of our biggest public health conundrums: vitamin D.
How much is too little (and how much is too much)?
We may be a sun-loving nation. But these are questions that have become more prominent in recent decades with lifestyle changes and shifting attitudes towards sun protection.
Skin can adapt to sunlight if you increase the exposure slowly, as opposed to going on a holiday and lying in the sun for five days straight.
"In the non-civilised days, or before we lived in cities, people would be slowly building up their defences against sunlight, starting from the end of winter," Professor of Endocrine Physiology at the University of Sydney Rebecca Mason told The Huffington Post Australia.
"Now, we tend to work indoors, not have much sun exposure and suddenly expect our bodies to cope when we go on holidays. There are a lot of negative things associated with that."
What is it?
Put simply, vitamin D is a hormone that is produced in the skin during sun exposure.
It is described as the link between skin and bone -- and this is where things get more complex.
"When it comes to bone health, vitamin D helps to absorb calcium from your diet and is needed to make bone," Mason said.
"We do know that people who have very low vitamin D levels develop a condition whereby some of the bone, without proper bone minerals, becomes soft and more likely to fracture."
Oral vitamin D (usually taken in conjunction with calcium) therefore helps to reduce the risks of fractures, falls and overall mortality -- although the effect is small.
While low vitamin D levels have been associated with a range of adverse health outcomes -- from increased risk of autoimmune diseases and infections -- Professor Mason says these links are unclear.
There have been some suggestions that the link between low vitamin D and other health outcomes could be a manifestation of low vitamin D being an index of ill health.
"It is very difficult to perform trials as vitamin D is normally present and isn't a therapeutic agent. The only things that have been shown in multiple clinical trials to be helped are falls, fractures and overall mortality -- particularly in older people," Mason said.
"There have been some suggestions that the link between low vitamin D and other health outcomes could be a manifestation of low vitamin D being an index of ill health."
Am I deficient?
According to the most recent ABS statistics, one in four Australian adults were vitamin D deficient in 2011-12. What is more important to note, however, is that most of these people had a mild deficiency (17 percent) or a moderate deficiency (6 percent), with less than 1 per cent being severely deficient.
These statistics were based off recommended levels in an Australian statement from the Medical Journal of Australia:
Total deficiency: <50 nmol/L
Mild deficiency: 30 – 49 nmol/L
Moderate deficiency: 13 – 29 nmol/L
Severe deficiency: <13 nmol/L
According to Professor Mason, the prevalence varies depending on state, location and season.
"The generally accepted level of deficiency is defined as below 50nmol/L. At the end of winter in states like Victoria and the ACT, the ABS have found that nearly one in two people were deficient in winter versus only about ten to 15 percent in summer," Mason said.
What happens if I am mildly deficient?
"If you have slightly low vitamin D, you won't notice much. You'll notice it much more if you are starting to reach low levels as you'll experience muscle weakness among other symptoms," Mason said.
"But with a mild deficiency, you are setting yourself up for adding to problems of muscle weakness and increased risks of falls and fractures as you get older. It's incremental."
Where do I get it?
The main source of vitamin D is UVB radiation from the sun, with adults unlikely to obtain more than five to ten percent of their vitamin requirement from their diet.
"Diet is not a particularly important source of vitamin D as it is not present in many foods. There's a little bit in things like eggs, mushrooms (if you stick them in the sun) and fish -- that is the best source," Mason said.
What is more important is calcium intake.
"Vitamin D helps to absorb calcium from your diet and having adequate calcium intake helps to protect your vitamin D from being broken down and being lost to the body," Mason said.
She recommends low-fat dairy products and foods such as tahini paste.
Nailing the balancing act
The production of vitamin D from exposure to sunlight can be influenced by a range of factors -- from your age to the time of day.
"Some sun exposure is good for us. The problem is that the same rays of UV that produce skin damage are the ones that produce vitamin D," Mason said.
Given Australia's high incidence of skin cancer, how do we toe the line between getting enough vitamin D for optimal health and protecting ourselves from over exposure?
According to Cancer Council guidelines, adequate vitamin D levels are reached through regular incidental exposure. When the UV index falls at 3 or above (during summer), this can be reached by spending a few minutes outside most days of the week.
During autumn and winter, when the UV Index falls below 3, time spent outdoors in the middle of the day with uncovered skin is recommended.
According to Mason, it comes down to sensible, short exposures.
"If you only expose your skin for a short period of time, there is not so much damage that it overwhelms the body's defences," she said.
"Once you get beyond that, you start to break down the extra vitamin D that is produced and that is less useful."
For longer periods in the sun, sensibly applied sun protection is key.
"Skin can adapt to sunlight if you increase the exposure slowly, as opposed to going on a holiday and lying in the sun for five days straight," Mason said.
Click below to follow HuffPost Australia on Snapchat!
ALSO ON HUFFPOST AUSTRALIA